Some of us are actively scavenging old questions and voting to close some of them as off-topic or unclear. This is great. If such a question has zero score, then it will be automatically deleted shortly after getting closed. However, if it has a positive score, it will not. Hence, if you think a question is bad, consider downvoting it as well.

Three examples that I had in the review queue right now one after another:

All three are from 2011--2013. All three had +1 score. All three were found by @Kjetil who voted to close, and all three got closed with me casting the last vote. However, nobody downvoted them until I did.

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I basically agree with @amoeba's reasoning. The rest of this post is just elaboration.

As a matter of expedience, I think it makes sense to down-vote old questions (posted more than 12 months ago) which are off-topic, low-quality, or too broad to be answerable. It takes five people to vote to close a question, but just one person to move a question from 0 score to -1 score. As a matter of policy, SE deletes questions with negative score and no answers (or no answers with positive score) on approximately a weekly basis. On a less-frequent basis, 0-score questions are deleted under certain conditions. It's not a very efficient use of resources to have 5 people involved in closing a years-old question that asks for code, or "which test do I run?".

There's a reason that I advocate down-voting old questions specifically. The closure queues are primarily oriented around giving recently-active users useful feedback on how to improve a question. Recent questions which might be off-topic or otherwise unsuitable should absolutely be placed in the queue so that active users can receive that feedback and improve their questions. We want to encourage new users.

In all cases, duplicate questions should be closed as duplicates, so that our canonical content is ranked highly by search engines & are otherwise more visible to users.

My overall philosophy regarding down-voting is that it should be rare and carefully considered. This is because down-voting is pretty strong negative feedback.

Down-voting that is not accompanied by a considered request for clarification or other context is inscrutable to the person who received the down-vote.